The instant invention relates to thermal printing and more particularly to a circuit for energizing a plurality of thermal print head elements on a thermal print head to effect printing on a thermally responsive sheet.
Thermal printing, wherein styli or print heads are selectively heated to produce markings on thermally sensitive sheets is well known and has been found to be effective for a wide variety of printing applications. In this regard, it is also well known that thermal print heads comprising large numbers of individually selectively energizable print head elements can be effectively utilized for producing well defined characters on thermally responsive sheets in thermal printing applications. Generally, thermal print heads of this type have comprised a body made of an electrically insulating material, such as glass or porcelain, and a plurality of print head elements which are disposed in closely spaced, but electrically insulated relation in a predetermined array on a printing surface of the body. For use of a thermal print head of this type, each of the print head elements thereof is electrically connected to energizing circuitry, and switch means is provided for each print head element so that each element is selectively energizable to produce a localized heated spot on the print head. Accordingly, a print head of this type can be used to produce images on a thermally sensitive sheet by selectively energizing various different combinations of elements on the print head for short periods of time to produce markings on the thermally sensitive sheet as the sheet is passed beneath the print head. Obviously, in this type of operation, the sizes of the individual markings produced on a thermally sensitive sheet depend on the sizes of the print head elements on the print head, the speed of the paper as it is passed beneath the print head, and the durations of the pulses during which the print head elements are energized. On the other hand, the darkness and uniformity of an image produced by a thermal print head are determined by the temperatures of the print head elements which, in turn, are related to the amounts of energy supplied to the print head elements when they are energized during the short pulses. However, it has been found that when a thermal print head comprising a number of print head elements is used in a continuous or prolonged printing operation, the overall temperature of the head can tend to gradually rise so that the print head elements require less energy to reach the same temperatures. As a result, if the amount of energy supplied to the print head elements of a print head is not decreased as the overall temperature of the print head increases, the images produced by the head can become increasingly dark and the sheet can even be burned by the head. Further, in this regard, the tendency of a thermal print head to build up heat is aggravated by the fact that the body portion of a print head must be made of an electrically insulating material, and as a rule, materials of this type inherently have relatively poor properties of thermal conductivity. In any event, as a result of these factors it has generally been found that it is necessary to provide some means for controlling the amount of energy which is supplied to the print head elements of a thermal print head as the temperature of the head is increased in order to produce images of consistent quality and darkness.
In addition to the above problem regarding heat buildup in thermal print heads, it has also been found that it is desirable to sequentially energize different groups of the print head elements on a thermal print head at different times during a printing cycle rather than to energize all of the specified elements on a print head at one particular time. In this regard, many thermal print heads contain large numbers of print head elements (up to 4,000 or more on large heads) and as a result, if all or a large number of the elements of a relatively large print head are energized at one time, a large current surge is produced which can cause a significant amount of interference in other circuitry in the general area. Accordingly, to overcome this problem, circuits have been developed which sequentially energize different groups of print head elements on a print head to different times during short printing cycles rather than energizing all of the selected elements at one time.
Accordingly, it has been found that in order for a thermal print head to be effective, it should be utilized in combination with circuitry which is operative for producing sequential pulses during a printing cycle and which is also operative for reducing the amount of energy which is supplied to the print head elements of a print head as the overall temperature of the print head is increased. Heretofore, this problem has been solved by providing circuitry which is operative for producing sequential pulses of uniform duration during a printing cycle, wherein the voltage applied to the print head elements during the pulses is reduced as the temperature of the print head is increased. However, it has been found that circuitry of this type is relatively complex and expensive, and therefore it has been found to be less than entirely satisfactory.
The instant invention provides a novel circuit which is operative for producing a plurality of sequential pulses during a printing cycle for energizing sequential groups of print head elements wherein the energy supplied to the printed elements is reduced as the temperature of the print head is increased. In this regard, however, the circuit of the instant invention is operative for controlling the energy supplied to the print head elements during the pulses by varying the durations of the pulses which are applied to the print head elements during a printing cycle rather than varying the voltage levels of the pulses. It has been found that this provides an effective means for controlling the amounts of energy supplied to the print head elements of a print head. Further, it has been found that the circuit of the instant invention is substantially simpler than the heretofore known circuits which have controlled the amounts of energy supplied to the print head elements of thermal print heads by controlling the amounts of voltage applied to the elements during sequential pulses and that therefore the circuit is adapted for substantially less expensive constructions than the heretofore available circuits for energizing print head elements.
The circuit of the instant invention which is operative for energizing a plurality of thermal print head elements on a thermal print head comprises first signal generating means for generating a signal having a frequency which is proportional to the value of a predetermined parameter, such as the temperature of the print head, second signal generating means which communicates with the first signal generating means and which is actuatable for generating a predetermined number of sequential pulses of substantially uniform duration, wherein the duration of the sequential pulses from the second signal generating means is proportional to the frequency of the signals from the first signal generating means, and means for actuating the second signal generating means in response to an input signal to the circuit and for deactuating the second signal generating means after a predetermined number of sequential pulses has been produced therefrom. Hence, when the first signal generating means is connected to a temperature sensing element on a print head, and the second signal generating means is connected to actuating or gating circuitry for the print head elements on the print head, each time an input signal is received by the actuating means, the circuit produces a cycle of pulses of substantially uniform duration and different groups of print head elements on the print head, are energized at different times during the cycle, but the duration of pulses in a particular cycle is proportional to the temperature of the print head as sensed by the sensing means. In other words, although the durations of all of the pulses in a particular cycle are substantially uniform, the durations of the pulses in different cycles vary in proportion to the temperature of the print head. Accordingly, when the circuit is connected to a device for producing input signals such as a microprocessor, and the print head elements are connected to the circuit through independently controllable switch means, the circuit can be effectively utilized for producing specified images of uniform darkness on a thermally responsive sheet. In addition, it will be understood that the circuit of the instant invention could also be used for controlling the energization of print head elements in response to other parameters than temperature. In any event, because of the relative simplicity of the circuit, it is effectively adapted for relatively inexpensive constructions which provides substantial economic advantages over the heretofore available circuits for energizing thermal print head elements.
Accordingly, it is a primary object of the instant invention to provide an economical circuit for controling the energization of a plurality of print head elements on a thermal print head.
Another object of the instant invention is to provide a circuit for controlling the energization of a plurality of print head elements on a print head with sequential pulses, wherein the durations of the pulses are proportional to the temperature of the print head.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.